There is a filter 100 in which a pipe P, which becomes the flow channel for cooling water of a radiator, is provided in such a way as to divide a primary side and a secondary side inside the pipe P (see FIG. 1 and FIG. 6 of Patent Document 1). However, in this type of filter 100, an effective area for filtering can be easily reduced.
There is a filter 200 hung in a radiator cap C, and placed inside the pipe P which becomes the flow channel for the cooling water of a radiator (see FIG. 7 of the Patent Document 1). However, such filter 200 does not completely divide the aforementioned pipe P, so that the filter 200 is not necessarily sufficient for filtering the cooling water.